Directional couplers of the strip line variety are finding more applications in various industries. For example, in the computer industry it has been found that directional couplers can be used to connect signals from transmission lines to other equipment such as computer terminals without having to interrupt the line to make the connection.
A strip line directional coupler is a device wherein two parallel adjacent printed circuit strip lines sandwiched between two ground planes are inductively and capacitively coupled so that the edges of the first pulse, of fast rise and fall time characteristics, propagating along one line, produces a positive pulse and a negative pulse in the other line. The lines are back coupled or directional in that the thus produced pulses propagate along the second line in a direction opposite to the direction in which the first pulse propagates along the first line. The energy transferred between the coupling segments of the two element directional coupler is affected by the various physical characteristics of the directional coupler such as the length, width and distance between the coupling segments. The signal of interest is coupled so as to travel in the opposite or backward direction of the signal on the first line. A parasitic signal is also produced travelling along the second line in what is called the forward direction. This parasitic forward coupled signal is usually of no importance since it is usually terminated in a resistance equal to the characteristic impedance of the line to which it is connected. However, problems arise in the situation where the forward coupled parasitic signals are directed into some other device such as a receiver, as in the case of co-pending U.S. Application Ser. No. 479,935, filed June 17, 1974, now U.S. Pat. No. 3,943,283, titled Bidirectional Signal Wire Data Transmission and Wrap Control, having the same inventors. In this co-pending application directional couplers are used to connect signals onto the same transmission line going in opposite directions. It will be appreciated, that the parasitic forward coupled signal, therefore, is travelling in the same direction as a backward coupled desired signal from another coupler. This undesired signal must be minimized so that it does not cause interference at the receiver or other device at which the desired backward coupled signal is received. In many situations, it is also desirable to put onto the transmission line a backward coupled signal having as great an amplitude as possible especially where the signal is to be transmitted over long distances and is therefore, subject to the attenuation of the transmission line and interference of noise signals which are invariably introduced.